Help with Messages+ errors

I just recently upgraded to a Samsung Galaxy S8 after having the S5 for several years. When I view my text messages in Message+ sometimes it shows an error message with an exclamation point inside a grey triangle in my thread. I know this is to indicate that a message did not send properly. When I first sent the original message, it got delivered to the recipient without an issue; however, I get the aforementioned error showing up in my main messaging thread several days later on a message that was sent over the network without incident. I end up having to scroll thru days of texts between my friend and I to remove the tagged message. Can someone please help me figure out what's going on within the messaging app and why I keep seeing that error message popping up  on a text that originally indicated it was sent, received and read by my friend? Its just rather irritating to have that icon pop up in a conversation that was already sent just fine several days ago. Thank you!  Nichole  

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